1. Field of the Invention
The present invention relates to a positive-electrode active material with high capacity and a lithium secondary battery including the same.
2. Related Art
Recently, lithium secondary batteries are used as a driving power source of vehicles, as well as for mobile electronic devices such as mobile phones, PDAs, laptop computers, and the like, so research is actively ongoing to improve the capacity of secondary batteries. In particular, in order to use a lithium secondary battery as a power source of a midsize and large devices such as HEV, PHEV, EV, and the like, an output of a certain level or higher, as well as high capacity, must be maintained in a state of charge (SOC) area in use in terms of safety and security, so in case of a secondary battery in which a rapid voltage drop occurs while it is discharged, the available SOC area is confined, limiting the secondary battery in its application as a driving power source. Thus, in order to use the lithium secondary battery for midsize and large devices, a material for the lithium secondary battery not causing a rapid output degradation over the wide SOC area and having high capacity is required to be developed.
The use of lithium metal, sulfur compounds, and the like, is considered as a negative electrode active material of the lithium secondary battery, and in most cases, a carbon material are used in consideration of security. When a carbon material is used as the negative electrode material, the capacity of the lithium secondary battery is determined by the capacity of positive electrodes, i.e., the amount of lithium ions contained in a positive electrode active material.
Meanwhile, a lithium-containing cobalt oxide (LiCoO2) is largely used as the positive electrode active material, and besides, the use of a lithium-containing manganese oxide such as LiMnO2 having a layered crystal structure, LiMN2O4 having a spinel crystal structure, and the like, and a lithium-containing nickel oxide (LiNiO2), or the like, has been considered.
Among the positive electrode active materials, LiCoO2, having excellent life span characteristics and charging/discharging efficiency, is commonly used, but disadvantageously, it has an inferior structural stability and is weak to price competitiveness due to a resource limitation of cobalt used as a raw material thereof. Thus, the use of LiCoO2 in large quantities as a power source in the sectors of electric automobiles, or the like, has a limitation.
The LiNiO2-based positive electrode active material is low-priced and exhibits battery characteristics of high discharge capacity, but it has a rapid phase transition of a crystal structure according to a change in volume accompanying in a charging and discharging cycle and its stability is drastically degraded when exposed to air and moisture.
Also, the lithium-containing manganese oxide, such as LiMnO2, or the like, is advantageous in that it has excellent thermal stability and is low-priced, but has drawbacks in that it has small capacity, poor cycle characteristics and poor high temperature characteristics.
Among the lithium manganese oxides, a spinel-based lithium manganese oxide exhibits a relatively smooth potential in a 4V range (3.7 to 4.3 V) and a 3V range, and when both ranges are all used, a great theoretical capacity of about 260 mAh/g or more (the theoretical capacity is about 130 mAh/g, respectively. in both 3V range and 4V range) can be obtained. However, it is known that cycle and storage characteristics of the spinel-based lithium manganese oxide are critically degraded in the 3V range, making it difficult to utilize the spinel-based lithium manganese oxide. Thus, when only the spinel-based lithium manganese oxide is used as the positive electrode active material, in the lithium secondary battery system in which a lithium source is dependent upon the positive electrode active material, there is no lithium source which may be used for charging and discharging in the 3V range, having a limitation that only a half of the available capacity is used. In addition, with the spinel-based lithium manganese oxide, a rapid voltage drop occurs between the 4V to 3V range to have a discontinuous voltage profile, potentially generating an output shortage in the range, and thus, the spinel-based lithium manganese oxide is not suitable to be used as a power source of midsize and large devices in the fields such as electric automobiles, or the like.
In order to complement the shortcomings of the spinel-based lithium manganese oxide and obtain excellent thermal stability of the manganese-based active material, a layered lithium manganese oxide has been proposed.
In particular, layered xLi2MnO3-(1-x)LiMO2 (0<x<1, M=Co, Ni, Mn, etc.) having greater content of manganese (Mn) than that of other transition metal(s) exhibits a quite high capacity in case of over-charging at a high voltage but disadvantageously has a great initial irreversible capacity. Various explanations are given to this phenomenon, and it is generally admitted as follows. Namely, as shown in a chemical formula below, two lithium ions and two electrons are eliminated along with oxygen gas from Li2MnO3 constituting the complex in a high voltage state of 4.5 V or higher based on a positive electrode potential in the event of charging, but only one lithium ion and one electron are reversibly inserted into the positive electrode in the event of discharging.
(Charging) Li2Mn4+O3→2Li+e−+½O2+Mn4+O2 
(Discharging) Mn4+O2+Li++e−→LiMn3+O2 
Thus, the initial charging/discharging efficiency of xLi2MnO3-(1-x)LiMO2 (0<x<1, M=Co, Ni, Mn, etc.), although it varies depending on the content of Li2MnO3 (x value), is lower than that of a general layered structure positive electrode material, e.g., LiCoO2, LiMn0.5Ni0.5O2, LiMn0.33Ni0.33Co0.33O2, etc.
In this case, in order to prevent lithium precipitation from a negative electrode in the initial cycle according to the great irreversible capacity of xLi2MnO3-(1-x)LiMO2, an excessive capacity of the negative electrode must be designed, resulting in a problem in that the actual reversible capacity may be possibly reduced. Thus, efforts have been made to regulate the irreversibility through surface coating, or the like, but the problem such as productivity, or the like, is yet to be solved. In addition, in case of a layered structure material, its stability is at issue.
Thus, the sole use of the conventionally known positive electrode active materials of the lithium secondary battery has shortcomings and limitations, so the mixture of the materials is required to be used, and in particular, in order for a lithium secondary battery to be used as a power source of a midsize and large devices, the lithium secondary battery is required not to have a rapid voltage drop range, namely, required to have an even profile in the entire SOC area, as well as having a high capacity, to thus have improved stability.